Network Working Group D. Thaler
Request for Comments: 4389 M. Talwar
Category: Experimental Microsoft
C. Patel
All Play, No Work
April 2006
Neighbor Discovery Proxies (ND Proxy)
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Bridging multiple links into a single entity has several operational
advantages. A single subnet prefix is sufficient to support multiple
physical links. There is no need to allocate subnet numbers to the
different networks, simplifying management. Bridging some types of
media requires network-layer support, however. This document
describes these cases and specifies the IP-layer support that enables
bridging under these circumstances.
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RFC 4389 ND Proxy April 20061. Introduction
In the IPv4 Internet today, it is common for Network Address
Translators (NATs) [NAT] to be used to easily connect one or more
leaf links to an existing network without requiring any coordination
with the network service provider. Since NATs modify IP addresses in
packets, they are problematic for many IP applications. As a result,
it is desirable to address the problem (for both IPv4 and IPv6)
without the need for NATs, while still maintaining the property that
no explicit cooperation from the router is needed.
One common solution is IEEE 802 bridging, as specified in [BRIDGE].
It is expected that whenever possible links will be bridged at the
link layer using classic bridge technology [BRIDGE] as opposed to
using the mechanisms herein. However, classic bridging at the data-
link layer has the following limitations (among others):
o It requires the ports to support promiscuous mode.
o It requires all ports to support the same type of link-layer
addressing (in particular, IEEE 802 addressing).
As a result, two common scenarios, described below, are not solved,
and it is these two scenarios we specifically target in this
document. While the mechanism described herein may apply to other
scenarios as well, we will concentrate our discussion on these two
scenarios.
1.1. SCENARIO 1: Wireless Upstream
The following figure illustrates a likely example:
| +-------+ +--------+
local |Ethernet | | Wireless | Access |
+---------+ A +-))) (((-+ +--> rest of network
hosts | | | link | Point |
| +-------+ +--------+
In this scenario, the access point has assigned an IPv6 subnet prefix
to the wireless link, and uses link-layer encryption so that wireless
clients may not see each other's data.
Classic bridging requires the bridge (node A in the above diagram) to
be in promiscuous mode. In this wireless scenario, A cannot put its
wireless interface into promiscuous mode, since one wireless node
cannot see traffic to/from other wireless nodes.
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IPv4 Address Resolution Protocol (ARP) proxying has been used for
some years to solve this problem without involving NAT or requiring
any change to the access point or router. In this document, we
describe equivalent functionality for IPv6 to remove this incentive
to deploy NATs in IPv6.
We also note that Prefix Delegation [PD] could also be used to solve
this scenario. There are, however, two disadvantages to this.
First, if an implementation already supports IPv4 ARP proxying (which
is indeed the case in a number of implementations today), then IPv6
Prefix Delegation would result in separate IPv6 subnets on either
side of the device, while a single IPv4 subnet would span both
segments. This topological discrepancy can complicate applications
and protocols that use the concept of a local subnet. Second, the
extent to which Prefix Delegation is supported for any particular
subscriber class is up to the service provider. Hence, there is no
guarantee that Prefix Delegation will work without explicit
configuration or additional charge. Bridging, on the other hand,
allows the device to work with zero configuration, regardless of the
service provider's policies, just as a NAT does. Hence bridging
avoids the incentive to NAT IPv6 just to avoid paying for, or
requiring configuration to get, another prefix.
1.2. SCENARIO 2: PPP Upstream
The following figure illustrates another likely example:
| +-------+ +--------+
local |Ethernet | | PPP link | |
+---------+ A +-----------+ Router +--> rest of network
hosts | | | | |
| +-------+ +--------+
In this scenario, the router has assigned a /64 to the PPP link and
advertises it in an IPv6 Router Advertisement.
Classic bridging does not support non-802 media. The PPP Bridging
Control Protocol [BCP] defines a mechanism for supporting bridging
over PPP, but it requires both ends to be configured to support it.
Hence IPv4 connectivity is often solved by making the proxy (node A
in the above diagram) be a NAT or an IPv4 ARP proxy. This document
specifies a solution for IPv6 that does not involve NAT or require
any change to the router.
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RFC 4389 ND Proxy April 20061.3. Inapplicable Scenarios
This document is not applicable to scenarios with loops in the
physical topology, or where routers exist on multiple segments.
These cases are detected and proxying is disabled (see Section 6).
In addition, this document is not appropriate for scenarios where
classic bridging can be applied, or when configuration of the router
can be done.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[KEYWORDS].
The term "proxy interface" will be used to refer to an interface
(which could itself be a bridge interface) over which network-layer
proxying is done as defined herein.
In this document, we make no distinction between a "link" (in the
classic IPv6 sense) and a "subnet". We use the term "segment" to
apply to a bridged component of the link.
Finally, while it is possible that functionality equivalent to that
described herein may be achieved by nodes that do not fulfill all the
requirements in [NODEREQ], in the remainder of this document we will
describe behavior in terms of an IPv6 node as defined in that
document.
3. Requirements
Proxy behavior is designed with the following requirements in mind:
o Support connecting multiple segments with a single subnet
prefix.
o Support media that cannot be bridged at the link layer.
o Do not require any changes to existing routers. That is,
routers on the subnet may be unaware that the subnet is being
bridged.
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o Provide full connectivity between all nodes in the subnet.
For example, if there are existing nodes (such as any routers
on the subnet) that have addresses in the subnet prefix,
adding a proxy must allow bridged nodes to have full
connectivity with existing nodes on the subnet.
o Prevent loops.
o Also work in the absence of any routers.
o Support nodes moving between segments. For example, a node
should be able to keep its address without seeing its address
as a duplicate due to any cache maintained at the proxy.
o Allow dynamic addition of a proxy without adversely
disrupting the network.
o The proxy behavior should not break any existing classic
bridges in use on a network segment.
3.1. Non-requirements
The following items are not considered requirements, as they are not
met by classic bridges:
o Show up as a hop in a traceroute.
o Use the shortest path between two nodes on different
segments.
o Be able to use all available interfaces simultaneously.
Instead, bridging technology relies on disabling redundant
interfaces to prevent loops.
o Support connecting media on which Neighbor Discovery is not
possible. For example, some technologies such as [6TO4] use
an algorithmic mapping from IPv6 address to the underlying
link-layer (IPv4 in this case) address, and hence cannot
support bridging arbitrary IP addresses.
The following additional items are not considered requirements for
this document:
o Support network-layer protocols other than IPv6. We do not
preclude such support, but it is not specified in this
document.
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o Support Redirects for off-subnet destinations that point to a
router on a different segment from the redirected host.
While this scenario may be desirable, no solution is
currently known that does not have undesirable side effects
outside the subnet. As a result, this scenario is outside
the scope of this document.
4. Proxy Behavior
Network-layer support for proxying between multiple interfaces SHOULD
be used only when classic bridging is not possible.
When a proxy interface comes up, the node puts it in "all-multicast"
mode so that it will receive all multicast packets. It is common for
interfaces not to support full promiscuous mode (e.g., on a wireless
client), but all-multicast mode is generally still supported.
As with all other interfaces, IPv6 maintains a neighbor cache for
each proxy interface, which will be used as described below.
4.1. Forwarding Packets
When a packet from any IPv6 source address other than the unspecified
address is received on a proxy interface, the neighbor cache of that
interface SHOULD be consulted to find an entry for the source IPv6
address. If no entry exists, one is created in the STALE state.
When any IPv6 packet is received on a proxy interface, it must be
parsed to see whether it is known to be of a type that negotiates
link-layer addresses. This document covers the following types:
Neighbor Solicitations, Neighbor Advertisements, Router
Advertisements, and Redirects. These packets are ones that can carry
link-layer addresses, and hence must be proxied (as described below)
so that packets between nodes on different segments can be received
by the proxy and have the correct link-layer address type on each
segment.
When any other IPv6 multicast packet is received on a proxy
interface, in addition to any normal IPv6 behavior such as being
delivered locally, it is forwarded unchanged (other than using a new
link-layer header) out all other proxy interfaces on the same link.
(As specified in [BRIDGE], the proxy may instead support multicast
learning and filtering, but this is OPTIONAL.) In particular, the
IPv6 Hop Limit is not updated, and no ICMP errors (except as noted in
Section 4.1.1 below) are sent as a result of attempting this
forwarding.
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When any other IPv6 unicast packet is received on a proxy interface,
if it is not locally destined then it is forwarded unchanged (other
than using a new link-layer header) to the proxy interface for which
the next hop address appears in the neighbor cache. Again the IPv6
Hop Limit is not updated, and no ICMP errors (except as noted in
Section 4.1.1 below) are sent as a result of attempting this
forwarding. To choose a proxy interface to forward to, the neighbor
cache is consulted, and the interface with the neighbor entry in the
"best" state is used. In order of least to most preferred, the
states (per [ND]) are INCOMPLETE, STALE, DELAY, PROBE, REACHABLE. A
packet is never forwarded back out the same interface on which it
arrived; such a packet is instead silently dropped.
If no cache entry exists (as may happen if the proxy has previously
evicted the cache entry or if the proxy is restarted), the proxy
SHOULD queue the packet and initiate Neighbor Discovery as if the
packet were being locally generated. The proxy MAY instead silently
drop the packet. In this case, the entry will eventually be re-
created when the sender re-attempts Neighbor Discovery.
The link-layer header and the link-layer address within the payload
for each forwarded packet will be modified as follows:
1) The source address will be the address of the outgoing
interface.
2) The destination address will be the address in the neighbor
entry corresponding to the destination IPv6 address.
3) The link-layer address within the payload is substituted with
the address of the outgoing interface.
4.1.1. Sending Packet Too Big Messages
Whenever any IPv6 packet is to be forwarded out an interface whose
MTU is smaller than the size of the packet, the ND proxy drops the
packet and sends a Packet Too Big message back to the source, as
described in [ICMPv6].
4.1.2. Proxying Packets with Link-Layer Addresses
Once it is determined that the packet is either multicast or else is
not locally destined (if unicast), the special types enumerated above
(ARP, etc.) that carry link-layer addresses are handled by generating
a proxy packet that contains the proxy's link-layer address on the
outgoing interface instead. Such link-layer addresses occur in the
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link-layer header itself, as well as in the payloads of some
protocols. As with all forwarded packets, the link-layer header is
new.
Section 4.1.3 enumerates the currently known cases where link-layer
addresses must be changed in payloads. For guidance on handling
future protocols, Section 7, "Guidelines to Proxy Developers",
describes the scenarios in which the link-layer address substitution
in the payload should be performed. Note that any change to the
length of a proxied packet, such as when the link-layer address
length changes, will require a corresponding change to the IPv6
Payload Length field.
4.1.3. IPv6 ND Proxying
When any IPv6 packet is received on a proxy interface, it must be
parsed to see whether it is known to be one of the following types:
Neighbor Solicitation, Neighbor Advertisement, Router Advertisement,
or Redirect.
4.1.3.1. ICMPv6 Neighbor Solicitations
If the received packet is an ICMPv6 Neighbor Solicitation (NS), the
NS is processed locally as described in Section 7.2.3 of [ND] but no
NA is generated immediately. Instead the NS is proxied as described
above and the NA will be proxied when it is received. This ensures
that the proxy does not interfere with hosts moving from one segment
to another since it never responds to an NS based on its own cache.
4.1.3.2. ICMPv6 Neighbor Advertisements
If the received packet is an ICMPv6 Neighbor Advertisement (NA), the
neighbor cache on the receiving interface is first updated as if the
NA were locally destined, and then the NA is proxied as described in
4.1.2 above.
4.1.3.3. ICMPv6 Router Advertisements
The following special processing is done for IPv6 Router
Advertisements (RAs).
A new "Proxy" bit is defined in the existing Router Advertisement
flags field as follows:
+-+-+-+-+-+-+-+-+
|M|O|H|Prf|P|Rsv|
+-+-+-+-+-+-+-+-+
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RFC 4389 ND Proxy April 2006
where "P" indicates the location of the Proxy bit, and "Rsv"
indicates the remaining reserved bits.
The proxy determines an "upstream" proxy interface, typically through
a (zero-configuration) physical choice dictated by the scenario (see
Scenarios 1 and 2 above), or through manual configuration.
When an RA with the P bit clear arrives on the upstream interface,
the P bit is set when the RA is proxied out all other ("downstream")
proxy interfaces (see Section 6).
If an RA with the P bit set has arrived on a given interface
(including the upstream interface) within the last 60 minutes, that
interface MUST NOT be used as a proxy interface; i.e., proxy
functionality is disabled on that interface.
Furthermore, if any RA (regardless of the value of the P bit) has
arrived on a "downstream" proxy interface within the last 60 minutes,
that interface MUST NOT be used as a proxy interface.
The RA is processed locally as well as proxied as described in
Section 4.1.2, unless such proxying is disabled as noted above.
4.1.3.4. ICMPv6 Redirects
If the received packet is an ICMPv6 Redirect message, then the
proxied packet should be modified as follows. If the proxy has a
valid (i.e., not INCOMPLETE) neighbor entry for the target address on
the same interface as the redirected host, then the Target Link-Layer
Address (TLLA) option in the proxied Redirect simply contains the
link-layer address of the target as found in the proxy's neighbor
entry, since the redirected host may reach the target address
directly. Otherwise, if the proxy has a valid neighbor entry for the
target address on some other interface, then the TLLA option in the
proxied packet contains the link-layer address of the proxy on the
sending interface, since the redirected host must reach the target
address through the proxy. Otherwise, the proxy has no valid
neighbor entry for the target address, and the proxied packet
contains no TLLA option, which will cause the redirected host to
perform Neighbor Discovery for the target address.
4.2. Originating Packets
Locally originated packets that are sent on a proxy interface also
follow the same rules as packets received on a proxy interface. If
no neighbor entry exists when a unicast packet is to be locally
originated, an interface can be chosen in any implementation-specific
fashion. Once the neighbor is resolved, the actual interface will be
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discovered and the packet will be sent on that interface. When a
multicast packet is to be locally originated, an interface can be
chosen in any implementation-specific fashion, and the packet will
then be forwarded out other proxy interfaces on the same link as
described in Section 4.1 above.
5. Example
Consider the following topology, where A and B are nodes on separate
segments which are connected by a proxy P:
A---|---P---|---B
a p1 p2 b
A and B have link-layer addresses a and b, respectively. P has
link-layer addresses p1 and p2 on the two segments. We now walk
through the actions that happen when A attempts to send an initial
IPv6 packet to B.
A first does a route lookup on the destination address B. This
matches the on-link subnet prefix, and a destination cache entry is
created as well as a neighbor cache entry in the INCOMPLETE state.
Before the packet can be sent, A needs to resolve B's link-layer
address and sends a Neighbor Solicitation (NS) to the solicited-node
multicast address for B. The Source Link-Layer Address (SLLA) option
in the solicitation contains A's link-layer address.
P receives the solicitation (since it is receiving all link-layer
multicast packets) and processes it as it would any multicast packet
by forwarding it out to other segments on the link. However, before
actually sending the packet, it determines if the packet being sent
is one that requires proxying. Since it is an NS, it creates a
neighbor entry for A on interface 1 and records its link-layer
address. It also creates a neighbor entry for B (on an arbitrary
proxy interface) in the INCOMPLETE state. Since the packet is
multicast, P then needs to proxy the NS out all other proxy
interfaces on the subnet. Before sending the packet out interface 2,
it replaces the link-layer address in the SLLA option with its own
link-layer address, p2.
B receives this NS, processing it as usual. Hence it creates a
neighbor entry for A mapping it to the link-layer address p2. It
responds with a Neighbor Advertisement (NA) sent to A containing B's
link-layer address b. The NA is sent using A's neighbor entry, i.e.,
to the link-layer address p2.
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The NA is received by P, which then processes it as it would any
unicast packet; i.e., it forwards this out interface 1, based on the
neighbor cache. However, before actually sending the packet out, it
inspects it to determine if the packet being sent is one that
requires proxying. Since it is an NA, it updates its neighbor entry
for B to be REACHABLE and records the link-layer address b. P then
replaces the link-layer address in the TLLA option with its own
link-layer address on the outgoing interface, p1. The packet is then
sent out interface 1.
A receives this NA, processing it as usual. Hence it creates a
neighbor entry for B on interface 2 in the REACHABLE state and
records the link-layer address p1.
6. Loop Prevention
An implementation MUST ensure that loops are prevented by using the P
bit in RAs as follows. The proxy determines an "upstream" proxy
interface, typically through a (zero-configuration) physical choice
dictated by the scenario (see Scenarios 1 and 2 above), or through
manual configuration. As described in Section 4.1.3.3, only the
upstream interface is allowed to receive RAs, and never from other
proxies. Proxy functionality is disabled on an interface otherwise.
Finally, a proxy MUST wait until it has sent two P bit RAs on a given
"downstream" interface before it enables forwarding on that
interface.
7. Guidelines to Proxy Developers
Proxy developers will have to accommodate protocols or protocol
options (for example, new ICMP messages) that are developed in the
future, or protocols that are not mentioned in this document (for
example, proprietary protocols). This section prescribes guidelines
that can be used by proxy developers to accommodate protocols that
are not mentioned herein.
1) If a link-layer address carried in the payload of the
protocol can be used in the link-layer header of future
messages, then the proxy should substitute it with its own
address. For example, the link-layer address in NA messages is
used in the link-layer header for future messages, and,
hence, the proxy substitutes it with its own address.
For multicast packets, the link-layer address substituted
within the payload will be different for each outgoing
interface.
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2) If the link-layer address in the payload of the protocol will
never be used in any link-layer header, then the proxy should
not substitute it with its own address. No special actions
are required for supporting these protocols. For example,
[DHCPv6] is in this category.
8. IANA Considerations
This document defines a new bit in the RA flags (the P bit). There
is currently no registration procedure for such bits, so IANA should
not take any action.
9. Security Considerations
Unsecured Neighbor Discovery has a number of security issues, which
are discussed in detail in [PSREQ]. RFC 3971 [SEND] defines security
mechanisms that can protect Neighbor Discovery.
Proxies are susceptible to the same kind of security issues that
plague hosts using unsecured Neighbor Discovery. These issues
include hijacking traffic and denial-of-service within the subnet.
Malicious nodes within the subnet can take advantage of this
property, and hijack traffic. In addition, a Neighbor Discovery
proxy is essentially a legitimate man-in-the-middle, which implies
that there is a need to distinguish proxies from unwanted man-in-
the-middle attackers.
This document does not introduce any new mechanisms for the
protection of proxy Neighbor Discovery. That is, it does not provide
a mechanism from authorizing certain devices to act as proxies, and
it does not provide extensions to SEND to make it possible to use
both SEND and proxies at the same time. We note that RFC 2461 [ND]
already defines the ability to proxy Neighbor Advertisements, and
extensions to SEND are already needed to cover that case, independent
of this document.
Note also that the use of proxy Neighbor Discovery may render it
impossible to use SEND both on the leaf subnet and on the external
subnet. This is because the modifications performed by the proxy
will invalidate the RSA Signature Option in a secured Neighbor
Discovery message, and cause SEND-capable nodes to either discard the
messages or treat them as unsecured. The latter is the desired
operation when SEND is used together with this specification, and it
ensures that SEND nodes within this environment can selectively
downgrade themselves to unsecure Neighbor Discovery when proxies are
present.
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In the following, we outline some potential paths to follow when
defining a secure proxy mechanism.
It is reasonable for nodes on the leaf subnet to have a secure
relationship with the proxy and to accept ND packets either from the
owner of a specific address (normal SEND) or from a trusted proxy
that it can verify (see below).
For nodes on the external subnet, there is a trade-off between
security (where all nodes have a secure relationship with the proxy)
and privacy (where no nodes are aware that the proxy is a proxy). In
the case of a point-to-point external link (Scenario 2), however,
SEND may not be a requirement on that link.
Verifying that ND packets come from a trusted proxy requires an
extension to the SEND protocol and is left for future work [SPND],
but is similar to the problem of securing Router Advertisements that
is supported today. For example, a rogue node can send a Router
Advertisement to cause a proxy to disable its proxy behavior, and
hence cause denial-of-service to other nodes; this threat is covered
in Section 4.2.1 of [PSREQ].
Alternative designs might involve schemes where the right for
representing a particular host is delegated to the proxy, or where
multiple nodes can make statements on behalf of one address
[RINGSIG].
10. Acknowledgements
The authors wish to thank Jari Arkko for contributing portions of the
Security Considerations text.
11. Normative References
[BRIDGE] T. Jeffree, editor, "Media Access Control (MAC) Bridges",
ANSI/IEEE Std 802.1D, 2004, http://standards.ieee.org/getieee802/download/802.1D-2004.pdf.
[ICMPv6] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[ND] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
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Appendix A: Comparison with Naive RA Proxy
It has been suggested that a simple Router Advertisement (RA) proxy
would be sufficient, where the subnet prefix in an RA is "stolen" by
the proxy and applied to a downstream link instead of an upstream
link. Other ND messages are not proxied.
There are many problems with this approach. First, it requires
cooperation from all nodes on the upstream link. No node (including
the router sending the RA) can have an address in the subnet or it
will not have connectivity with nodes on the downstream link. This
is because when a node on a downstream link tries to do Neighbor
Discovery, and the proxy does not send the NS on the upstream link,
it will never discover the neighbor on the upstream link. Similarly,
if messages are not proxied during Duplicate Address Detection (DAD),
conflicts can occur.
Second, if the proxy assumes that no nodes on the upstream link have
addresses in the prefix, such a proxy could not be safely deployed
without cooperation from the network administrator since it
introduces a requirement that the router itself not have an address
in the prefix. This rules out use in situations where bridges and
Network Address Translators (NATs) are used today, which is the
problem this document is directly addressing. Instead, where a
prefix is desired for use on one or more downstream links in
cooperation with the network administrator, Prefix Delegation [PD]
should be used instead.
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